Detection of He II reionization in the Sloan Digital Sky Survey quasar sample

The expansion of the universe leads to a rapid drop in the hydrogen Lyalpha effective optical depth of the intergalactic medium (IGM)(τ) over bar (eff) proportional to (1+z)(3.8), between redshifts 4 and 3. Measurements of the temperature evolution of the IGM and of the He II opacity both suggest that He II reionizes in this redshift range. We use hydrodynamical simulations to show that the observed temperature increase associated with He II reionization leads to a relatively sudden decrease in (τ) over bar (eff) around the reionization epoch of approximate to10%. We find clear evidence for such a feature in the evolution of (τ) over bar (eff) determined from a sample of similar to1100 quasars obtained from the Sloan Digital Sky Survey. He II reionization starts at redshift approximate to3.4 and lasts for Deltazapproximate to0.4. The increase in the IGM temperature also explains the widths of hydrogen absorption lines as measured in high-resolution spectra.